Direct observation of the conformational states of formin mDia1 at actin filament barbed ends and along the filament

Using electron microscopy, two conformations of the mDia1 formin FH2 domains were directly identified in interaction with the barbed ends of actin. These conformations agree with the speculated open and closed conformations of the “stair-stepping” model. In addition, single FH2 dimers encircling the core of actin filaments were identified.